Process Variables

It is possible to play on two variables to optimize the application of precipitation by nitrogen removal: the stoichiometric coefficient and pH. The composition of the waste water determines the possibilities of finding a good solution to a particular waste water problem by the use of these two variables.

The optimum pH for precipitation of magnesium-ammonium-phosphate may be found by use of double logarithmic diagrams, as presented in Section 11.1. The method is best illustrated by presentation of a concrete case study. Let us consider a waste water with the concentration of ammonium at 2mmol/1 and of phosphate at 0.3 mmol /I, corresponding to municipal waste water. Let us furthermore presume that we use 0.02 mol/l magnesium for the precipitation. What, under these circumstances, is the optimum pH? Several processes are interacting: the acid-base reactions of phosphate. Phosphoric acid has three pKa-values: 2.1, 7.2 and 12.3; see also Fig. 11.2. Ammonium has a pKa-value of 9.25. The solubility product of magnesium-ammonium-phosphate is 10"12-6 . Let us also assume that the ionic strength is too small to have any significant influence on the equilibrium constants.

Figure 11.7. is a double logarithmic diagram of phosphate and ammonium in the actual concentrations. It can be seen on the diagram that the product of ammonium and phosphate reaches its highest value at about pH = 10.7, which is the optimum for the precipitation, when the concentrations of free magnesium ions are accounted for.

Figure 11.8 is constructed from 11.7. The product of the phosphate and ammonium concentrations are plotted versus pH and on the diagram shows, where the product exceeds 10'10-6, corresponding to a magnesium concentration of 10 mmol /1. It is possible to obtain an effective precipitation but the stoichiometric ratio between phosphate and ammonium must of course be 1:1 to assure that both components are readily precipitated. It Implies, that if 1.7 mmol/l phosphate and 10

mmol /I magnesium are added, an almost complete removal of ammonium and phosphate is possible.

Figure 11.7. Double logarithmic plot of ammonium and phosphate in the concentrations found generally in municipal waste water.

It is not possible theoretically to calculate the optimum condition for precipitation of proteins. It is necessary to make laboratory experiments to arrive at the relationship between removal efficiency on the one hand and pH, the amount and type of precipitant on the other; see for instance Jorgensen (1989). Several précipitants in combinations with at least a few polyflocculants must be tested at 3 or more different pH values. The settling rate is observed and used, as will be shown in Section 11.3, to design the sedimentation unit, while plots of the type shown in Figs. 11.9 -10 are used to determine which precipitant to use, in which amount and àt which pH to obtain the best precipitation. As seen in Fig. 11.9 the obtained COD of the effluent is plotted versus the amount of precipitant added for three different précipitants. BOD5 or the permanganate number or the total nitrogen concentration could of course also be used.

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